Downhole drilling operations in an oil or gas well normally involve the circulation of fluid, to wash cuttings away from the drill bit at the bottom of the hole, and to return the cuttings to the surface. US 2014/0099447 discloses a valve used in such operations which is useful for understanding the present method and apparatus. Valves are normally operated by landing a plug, for example a ball, on a seat (e.g. a ball seat), or shearing a pin, to open a radial outlet port to an annulus. Fluid can circulate through the open outlet port to the annulus outside the valve, which can be helpful in clearing the annulus of cuttings or other debris. The plug is generally expelled from the valve seat either under the action of fluid pressure alone, or in tandem with a smaller plug that is dropped after the first plug, and which has smaller dimensions, thus allowing the second plug to pass freely through the seat.
U.S. Pat. No. 7,681,650 and WO 01/90529 are also useful for understanding the present apparatus and method.